1. Field of the Invention
The invention relates to a piezoelectric device and particularly relates to a surface mount piezoelectric device, which suppresses frequency variation caused by humidity permeating an encapsulation resin from the external atmosphere, and a fabricating method of the surface mount piezoelectric device.
2. Description of the Related Art
As a typical example of crystal oscillator, surface mount piezoelectric devices (referred to as crystal oscillator or oscillator in this specification), are extensively used as a signal reference source of frequency, time, and etc. in portable electronic devices. One type of the surface mount piezoelectric devices is an H-shaped surface mount piezoelectric device. The H-shaped surface mount piezoelectric device includes a container body (package) that is shaped like the English letter H in a longitudinal cross-sectional view thereof, wherein the upper concave of the H-shaped container body is a top cavity for disposing a crystal resonator, and the lower concave of the H-shaped container body is a bottom cavity for mounting an IC chip that forms a temperature control circuit or an oscillation circuit. The IC chip mounted to the container body is a surface mount type IC chip, and the piezoelectric device (oscillator), mounted with the aforementioned crystal resonator and IC chip, is the surface mount piezoelectric device.
FIG. 8 is a schematic view depicting an exemplary configuration of a conventional H-shaped surface mount piezoelectric device. In this configuration, the container body 1 is a multi-layer structure formed of ceramic constructive layers 1a, 1b, 1c, and 1d. In this example, a printed circuit is respectively formed on one or two sides of each of the constructive layers 1a, 1b, 1c, and 1d, that is, HTCC (high temperature cofired ceramics). However, the number of the ceramic constructive layers is not limited to the above, and the container body 1 may include only one layer in other examples. In that case, the container body 1 is not a multi-layer structure. Moreover, LTCC (low temperature cofired ceramics) is also applicable according to the wiring material that is formed in the ceramics.
A crystal resonator 4 is disposed in the top cavity 2 that constitutes a part of the container body 1 of the H-shaped surface mount piezoelectric device. The crystal resonator 4 is mostly a crystal oscillator, and a terminal part of the crystal resonator 4 is connected with a wiring formed on an interior surface of the top cavity 2 via an electrically-conductive adhesive agent 4a. The crystal resonator 4 is fixed to the container body 1 at a side of the crystal resonator 4 by the connection part. Moreover, the top cavity 2 is sealed from the external atmosphere by a cover body 6 that is formed of a metal material and adhered to the container body 1 via a metal ring 5 disposed between edges of the container body 1 and the top cavity 2. Generally, the metal ring 5 and the cover body 6 are welded to the container body 1 via a silver solder 5a by using Kovar, which has a coefficient of thermal expansion close to that of ceramics. The cover body 6 is fixed to the metal ring 5 by seam welding.
An IC chip 8, which includes a circuit for generating a desired frequency output based on an oscillation output of the crystal resonator 4, is surface-mounted onto a bottom surface of the bottom cavity 3, which constitutes a part of the container body 1. In addition, the IC chip 8 can also serve as a temperature control circuit. The IC chip 8 is surface-mounted by a means, such as ultrasonic bonding, and using a flip chip bonder or a chip mounter to fix bumps 9, formed of solder balls on a mounting surface (one side) of the IC chip 8, to terminal pads 7a of a circuit wiring pattern 7 formed on the bottom surface of the bottom cavity 3.
The circuit wiring patterns of the container body 1 at the side of the IC chip 8 and the side of the crystal resonator 4 are connected with each other via a via hole (not shown in the figure) that is configured in the constructive layer 1b of the container body 1. In addition, a plurality of terminals 10 are disposed on edges of the bottom cavity 3 for connecting with an external circuit via surface mounting. The terminals 10 and the circuit wiring patterns of the container body 1 are also connected via a through hole (not shown in the figure) configured in the constructive layers of the container body 1.
An organic resin 13 fills the bottom cavity 3 where the IC chip 8 is mounted, including a gap between the bottom cavity 3 and the mounting surface of the IC chip 8, for protecting the IC chip 8 from damage caused by an external shock. Generally an epoxy resin is used as the organic resin 13.